Studies on lichen-dominated systems. XVIII. Morphological control of evaporation in lichens

Abstract

The current belief that lichens are incapable of controlling their water relations is based largely on evidence collected from experiments testing for active metabolic controls only. In this study, controlled wind tunnel experiments designed to test for any passive morphological control of evaporation in four lichen species show that such mechanisms are important to the water relations of the plants and thus suggest that the observed morphological variability in lichens is of great ecological significance.Time-dependent and moisture-content-dependent changes in evaporation rate were examined using samples of lichen thalli from a number of different arctic and subarctic populations of Alectoria ochroleuca (Hoffm.) Massal., Alectoria nitidula (Th.Fr) Vain, Cetraria nivalis (L.) Ach., and Cladina alpestris (L.) Harm. The work has shown that the observed interspecific and intraspecific variability in evaporative resistance found in these plants is influenced by three separate morphological characteristics: variability in surface area to weight ratio; variability in thallus shape; and variability in the degree of clumping of thalli. When other aspects of the ecology and physiology of the test lichens were considered, it became clear that these lichens exhibited a potential to modify their morphological characteristics over long periods of time in such a way that greater photosynthetic productivity was assured.It was concluded that although the control of water relations in lichens is of a different nature from higher plants, the control is present nonetheless.

Patterns of CO2gas-exchange and thallus water content in Arctic lichens along a ridge profile near Ny Ålesund, Svalbard

“…-C. nivalis dried out faster than C. delisei after a precipitation event or artificial irrigation. Desiccation of lichens is dependent on thallus-morphology and the microtopography of their habitat (Larson & Kershaw 1976;Kershaw 1985). Denser mats and more finely branched thalli, like C. delisei, will have a higher surface to weight ratio (A/W-ratio) and hence a greater evaporation resistance and water holding capacity (Larson & Kershaw 1976;Larson 1981).…”

“…The lichen cushions dried slower than the single thallus pieces, again due to the higher water holding capacity and higher evaporation resistance of cushions compared with that of single thalli. Larson & Kershaw (1976) showed the contribution of area to weight ratio, thallus shape and degree of clumping in the control of evaporation rate in lichens. For example, the area to weight ratio (cm'/gDW) of the loosely growing cushions of C. nivalis (345-370) was less than that of Cladonia stellaris (420-625), which grows in denser clumps.…”

Impacts of reindeer grazing on soil properties on Finnmarksvidda, northern Norway

“…3), that reindeer grazing also has a significant impact on soil water budget. In general, a well developed lichen surface acts as an effective mulch and elevates the soil moisture status throughout the summer period by preventing evaporation (Larson & Kershaw, 1976;Kershaw, 1977;Kershaw, 1985). In subarctic welldrained terrain acute summer droughts are quite common which are assumed to be lethal to fine roots of trees and dwarf shrubs, that develop close to the soil surface (Crittenden, 2000).…”

Carbon Dioxide Exchange in Lichens: Relationship Between the Diffusive Resistance of Carbon Dioxide and Water Vapour

“…However, for this reason, at high water contents H o would be expected to be identical to, or lower than, the values at the highest water content measured. The results of Harris (1976), Larson & Kershaw (1976) and Larson (1979) T h a i l u s w a t e r c o n t e n t i m g w a t e r m g^' d r y w e i g h t ;…”